1. Field of the Invention
This invention relates to a catalyst system which is a combination of at least one unbridged metallocene catalyst and at least one heterogeneous catalyst. Combining at least one unbridged metallocene catalyst with the heterogeneous catalyst affects the molecular weight (M.sub.w), the molecular weight distribution (MWD or M.sub.w /M.sub.n) and the melt flow index of the resulting polymer. Combining at least one unbridged metallocene catalyst and at least one bridged metallocene catalyst with the heterogeneous catalyst affects the xylene solubles in addition to the molecular weight, the molecular weight distribution and melt flow index of the resulting polymer.
2. Description of the Prior Art
It is known that two or more homogeneous catalysts, such as those based on metallocene compounds, may be combined to effect properties, such as molecular weight distribution. U.S. Pat. No. 4,530,914 discloses use of a catalyst system comprising two or more metallocenes in the polymerization of a-olefins, primarily ethylene, to obtain a broad molecular weight distribution. The metallocenes each have different propagation and termination rate constants. The metallocenes are mixed with an alumoxane to form the catalyst system.
It is also known that metallocenes may be affixed to a support to simulate a heterogeneous catalyst. U.S. Pat. No. 4,808,561 discloses reacting a metallocene with an alumoxane and forming a reaction product in the presence of a support. The support is a porous material like talc, inorganic oxides such as Group IIA, IIIA IVA or IVB metal oxides like silica, alumina, silica-alumina, magnesia, titania, zirconia and mixtures thereof, and resinous material such as polyolefins, e.g., finely divided polyethylene. The metallocenes and alumoxanes are deposited on the dehydrated support material.
In U.S. Pat. No. 4,701,432 a support is treated with at least one metallocene and at least one non-metallocene transition metal compound. To form a catalyst system a cocatalyst comprising an alumoxane and an organometallic compound of Group IA, IIA, IIB and IIIA is added to the supported metallocene/non-metallocene. The support is a porous solid such as talc or inorganic oxides or resinous materials, preferably an inorganic oxide, such as silica, alumina, silica-alumina, magnesia, titania or zirconia, in finely divided form. By depositing the soluble metallocene on the support material it is converted to a heterogeneous supported catalyst. The transition metal compound, such as TiCl.sub.4, is contacted with the support material prior to, after, simultaneously with or separately from contacting the metallocene with the support.
It is known that Cp.sub.2 TiCl.sub.2 in the presence of alkylaluminum compounds polymerizes ethylene but not propylene whereas in the presence of methylalumoxane (MAO) Cp.sub.2 TiCl.sub.2 polymerizes propylene also to produce atactic polypropylene. Combination of dimethyl titanocene and its Cp-substituted analogues and TiCl.sub.3 for propylene polymerizations has been reported in U.S. Pat. No. 2,992,212 and in "Thermoplastic Elastomers Based on Block Copolymers of Ethylene and Propylene", G. A. Lock, Advances in Polyolefins, p. 59-74, Raymond B. Seymour, Ed. MAO was not used in this polymerization.
It would be advantageous to change polymer properties in a polymerization processes using a conventional supported Ziegler-Natta catalyst by the combination with at least one metallocene catalyst which may interact or modify the Ziegler-Natta catalyst in addition to producing polymer.